Distribution and Spatial Organization of Dopaminergic Interplexiform Cells in the Rat Retina

Since the discovery of intrinsic retinal dopamine (DA)-neurons (Malmfors, 1963), a number of physiological roles in retinal neurotransmission have been established for DA (see Ehinger, 1983, for review). The best documented at the level of cellular and molecular biology is unquestionably the uncoupling action of DA on horizontal cell gap junctions via a D1 receptor mechanism. The DA released by light stimulation (flashes or light adaptation from drakness) contributes to the regulation of horizontal cell receptive field size which is a mechanism coding for contrast sensitivity. The narrowing of receptive fields due to the diminished ion permeability of gap junctions results in an enhanced perception of contrast which is likely to improve visual acuity. The anatomical support for such an action of DA at the level of the outer plexiform layer would be the so-called interplexiform cells (Gallego, 1971), which contain DA in fish (Ehinger et al., 1969), because these cells, located in the amacrine cell layer together with DA- amacrines, send processes to both the inner (IPL) and outer (OPL) plexiform layers. Although the uncoupling action of DA on horizontal cells has been demonstrated in the retina of cold-blooded animals (Piccolino et al., 1984; Teranishi et al., 1983), there are supporting evidences for the occurence of such a mechanism in mammals and man whose retina also contains DA-interplexiform cells (DA-IPcs) (Hokoc and Mariani, 1987; Negishi et al., 1985; Nguyen-Legros et al., 1981, 1984; Oyster, 1985; Ryan and Hendrickson, 1987; Versaux-Botteri et al., 1984).